Added GroupCategoriesSerializer

• Added:
- GroupCategoriesSerializer

app/serializers/rest/group_categories_serializer.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+class REST::GroupCategoriesSerializer < ActiveModel::Serializer
+  attributes :id, :text, :created_at
+
+  def id
+    object.id.to_s
+  end
+end